Processes for preparing printing formes for planography by electrophotography have been known, and the printing formes are usually obtained according to the following method: First, photoconductive layers are uniformly charged, subjected to imagewise exposure, and developed through a wet process or a dry process to obtain toner images, followed by fixing them. Non-image portions are then made hydrophilic by desensitizing treatment to obtain the printing formes for planography. In the printing formes for planography, scumming on the non-image portions (referred to as "background scumming") must be avoided. However, in the printing formes for planography thus obtained, background scumming development has not actually been avoided completely. Particularly, the use of an exhausted desensitizing solution employed repeatedly, color inks in printing, and neutral paper as printed paper has frequently resulted in development of background scumming.
It is effective against such background scumming to repeat the desensitizing treatment twice or more. However, this method increases steps to lower operation efficiency, and cannot be adopted in automatic printing machines which are recently widely used where etching processors for the desensitizing treatment are integrated into printers. Furthermore, this method also has a problem of impairing inking property on image portions.
As other methods to prevent background scumming, for example, JP-B-50-31011 (the term "JP-B" as used herein means an "examined published Japanese patent publication"), JP-A-54-20735 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-58-68046 disclose that improvements in resin binders used for photoconductive layers are effective against background scumming. However, actual detailed examination of the binders shows that the effect of preventing scumming has not been fully satisfactory.